Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. Such elements are described in, for example, Research Disclosure, June 1978, Item No. 17029 and U.S. Pat. Nos. 3,080,254, 3,457,075 and 3,933,508.
The aforesaid thermally processable imaging elements are often provided with an overcoat layer and/or a backing layer, with the overcoat layer being the outermost layer on the side of the support on which the imaging layer is coated and the backing layer being the outermost layer on the opposite side of the support. Other layers which are advantageously incorporated in thermally processable imaging elements include subbing layers and barrier layers.
To be fully acceptable, a protective overcoat layer for such imaging elements should: (a) provide resistance to deformation of the layers of the element during thermal processing, (b) prevent or reduce loss of volatile components in the element during thermal processing, (c) reduce or prevent transfer of essential imaging components from one or more of the layers of the element into the overcoat layer during manufacture of the element or during storage of the element prior to imaging and thermal processing, (d) enable satisfactory adhesion of the overcoat to a contiguous layer of the element, (e) be free from cracking and undesired marking, such as abrasion marking, during manufacture, storage, and processing of the element, (f) provide adequate conveyance characteristics during manufacture and processing of the element, (g) not allow blocking, adhering or slippage of the element during manufacture, storage, or processing, (h) not induce undesirable sensitometric effects in the element during manufacture, storage or processing and (i) provide a moisture barrier to the imaging layer.
A backing layer also serves several important functions which improve the overall performance of thermally processable imaging elements. For example, a backing layer serves to improve conveyance, reduce electrostatic charging and eliminate formation of Newton Rings.
A particularly preferred overcoat for thermally processable imaging elements is an overcoat comprising poly(silicic acid) as described in U.S. Pat. No. 4,741,992, issued May 3, 1988. Advantageously, water-soluble hydroxyl-containing monomers or polymers are incorporated in the overcoat layer together with the poly(silicic acid). The combination of poly(silicic acid) and a water-soluble hydroxyl-containing monomer or polymer that is compatible with the poly(silicic acid) is also useful in a backing layer on the side of the support opposite to the imaging layer as described in U.S. Pat. No. 4,828,971, issued May 9, 1989.
U.S. Pat. No. 4,828,971 explains the requirements for backing layers in thermally processable imaging elements. It points out that an optimum backing layer must:
(a) provide adequate conveyance characteristics during manufacturing steps, PA1 (b) provide resistance to deformation of the element during thermal processing, PA1 (c) enable satisfactory adhesion of the backing layer to the support of the element without undesired removal during thermal processing, PA1 (d) be free from cracking and undesired marking, such as abrasion marking during manufacture, storage and processing of the element, PA1 (e) reduce static electricity effects during manufacture, processing and use, and PA1 (f) not provide undesired sensitometric effects in the element during manufacture, storage or processing. PA1 (1) a support; PA1 (2) a thermographic or photothermographic imaging layer on one side of the support; PA1 (3) a protective overcoat layer which is an outermost layer on the same side of the support as the imaging layer; PA1 (4) a backing layer which is an outermost layer located on the side of the support opposite to the imaging layer; and PA1 (5) an electroconductive agent in at least one layer of the imaging element; PA1 (1) Electroconductive layers comprised of electrically-conductive metal-containing particles dispersed in a polymeric binder. Examples of useful electrically-conductive metal-containing particles include donor-doped metal oxide, metal oxides containing oxygen deficiencies and conductive nitrides, carbides or borides. Specfic examples of particularly useful particles include conductive TiO.sub.2, SnO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, In.sub.2 O.sub.3, ZnO, TiB.sub.2, ZrB.sub.2, NbB.sub.2, TaB.sub.2, CrB.sub.2, MoB, WB, LaB.sub.6, ZrN, TiN, TiC, WC, HfC, HfN and ZrC. PA1 (2) Electroconductive layers composed of a vapor-deposited metal such as silver, aluminum or nickel; PA1 (3) Electroconductive layers composed of binderless electrically-semiconductive metal oxide thin films formed by oxidation of vapor-deposited metal films as described in U.S. Pat. No. 4,078,935. PA1 (4) Electroconductive layers composed of conductive polymers such as, for example, the cross-linked vinylbenzyl quaternary ammonium polymers of U.S. Pat. No. 4,070,189 or the conductive polyanilines of U.S. Pat. No. 4,237,194. PA1 (1) FC-135 fluorosurfactant which is available from Minnesota Mining and Manufacturing Company and is a cationic fluorosurfactant in which the active agent is comprised of flurorinated alkyl quaternary ammonium iodides such as the iodide of the formula: EQU C.sub.8 F.sub.17 --SO.sub.2 NHCH.sub.2 CH.sub.2 CH.sub.2 N.sup.+ (CH.sub.3).sub.3 I.sup.- PA1 (2) FC-431 fluorosurfactant which is available from Minnesota Mining and Manufacturing Company and is a non-ionic fluorosurfactant comprised of a mixture of fluoroaliphatic polymeric esters. PA1 (3) Lodyne S-100 fluorosurfactant which is a non-ionic fluorosurfactant available from Ciba Geigy Corporation and is a mixture of compounds of the formula: EQU R.sub.f --(CH.sub.2).sub.2 --S--CH(CO.sub.2 H)CH.sub.2 CONH(CH.sub.2).sub.3 --N(CH.sub.3).sub.2 PA1 (4) Zonyl FSN fluorosurfactant which is a non-ionic fluorosurfactant available from DuPont and has the formula: EQU R.sub.f --CH.sub.2 CH.sub.2 --O--(CH.sub.2 CH.sub.2 O).sub.x --H
With photothermographic elements, it is usually necessary to produce a "duplicate image" of that on the imaging element for low cost dissemination of the image. The duplication process is typically a "contact printing" process where intimate contact between the photothermographic imaging element and the duplication imaging element is essential. Successful duplication of either continuous rolls or cut sheets is dependent on adequate conveyance of the imaging element through the duplication equipment without the occurrence of slippage or sticking of the protective overcoat layer of the photothermographic imaging element in relation to any of (1) the duplication equipment, (2) the duplication imaging element or (3) the backing layer of subsequent portions of the photothermographic imaging element (adjacent convolutions of the photothermographic imaging element if in a continuous roll or adjacent "cut sheets" in a stacking configuration). The latter of these phenomena is often referred to as "blocking".
The addition of matte particles in the protective overcoat layer is commonly used to prevent adhering or "blocking" between the protective overcoat layer and adjacent backing layer with which it is in intimate contact during manufacture, storage, processing and photo duplication. Furthermore, the matte particles are necessary to impart anti-frictional characteristics to the protective overcoat layer to achieve proper conveyance without sticking, blocking or slippage during the duplication process. The amount and particle size must be controlled as the wrong particle size and/or amount can cause both conveyance and duplicate image quality problems.
The photothermographic imaging element is typically viewed at magnification ratios as high as 100.times.. The matte particle in the protective overcoat layer, if too large, can negatively alter the appearance of the image in the photothermographic imaging element layer when viewed at magnification larger than 1.times.. This altered image can further be transferred through the duplication process as well as a tertiary transformation of the image to paper through contact printing, electrophotographic processes, thermal printing or similar processes.
In order to provide antistatic protection, it is desirable to incorporate an electroconductive agent in at least one layer of a thermally-processable imaging element. The electroconductive agent can be present, for example, in the protective overcoat layer, in the backing layer or in a separate electroconductive layer. Incorporation of an electroconductive agent in a separate electroconductive layer is described in U.S. Pat. No. 5,310,640. Incorporation of an electroconductive agent in an outermost layer of the thermally-processable imaging element which also performs other functions is described in copending commonly-assigned U.S. Pat. No. 5,547,821, "Thermally Processable Imaging Element Comprising A Surface Layer That Is Electroconductive" by Sharon M. Melpolder et al, granted Aug. 20, 1996.
The protective overcoat layer of a thermally-processable imaging element can develop a triboelectric charge of a given polarity and magnitude as a consequence of contact with and separation from other materials during normal use in the imaging system for which it is intended. In a similar manner, the backing layer can also develop a triboelectric charge. The triboelectric charge produces an electric field of a given polarity and magnitude. The electroconductive layer of an adjacent thermally-processable element can be influenced by this electric field and charge migration can occur so as to concentrate charged ions within the electroconductive layer in proximity to the charged surface. This results in an electrostatic force of attraction between the two adjacent imaging elements. This force of attraction may be sufficiently strong as to cause adhesion of the two imaging elements. The adhesion can cause serious difficulties with regard to conveyance of the imaging elements through the imaging system as well as serious difficulties in manual handling operations such as stacking, sorting and filing.
It is toward the objective of providing an improved thermally-processable imaging element, that overcomes the problems hereinabove described, that this invention is directed.